Subsea power distribution is currently under development, where also switchgear is seen as an important component to realize reliable operations. A circuit breaker for an AC power supply is often constructed as a vacuum circuit breaker or a gas-filled circuit breaker to control or eliminate the electric arc that usually occurs when breaking a large electric current. However, such circuit breakers, when connected in circuits including a voltage source converter (such as for operation of electric motors) require additional over-voltage protection equipment for the converter. This normally requires a chopper and a bypass.
In some DC power systems it has been known to use no-load switching devices. In U.S. Pat. No. 7,067,940, entitled “Submarine branching unit having asymmetrical architecture” and assigned to Pirelli, a submarine branching unit of a submarine optical cable system is described. Relays are used to configure and switch DC power flowing in different cables. The relays are arranged to switch current at zero current conditions. EP2293407, entitled “Power transmission and distribution systems” and assigned to Converteam Technologies describes a subsea DC power and transmission distribution system which includes DC/DC converter units that can be switched at no-current or at zero voltage.
One application for a circuit breaker in which a vacuum based design creates difficulties is in a subsea application. Apart from the sealing issues to prevent penetration of sea water, there arises the issue of increased ambient pressure at significant depths under the surface of the sea. Thus a subsea implementation of switchgear can be found where a thick tank is constructed to keep a close-to 1 atm (atmosphere) pressure inside the vessel in which a normal off-the-shelf circuit breaking chamber is installed. For example, in U.S. Pat. No. 8,111,502, entitled “Arrangement adapted for a subsea application”, and assigned to Vetco Gray Scandinavia AS, an arrangement for AC electric power distribution is described which is enclosed in such a watertight casing. The arrangement uses an AC voltage regulator to reduce a voltage to a No-load level during switching by using a magnetic flux adaption method.
For greater depths, such a construction becomes extremely heavy and inconvenient. At a depth of 3000 meters the pressure on any enclosure is around 300 atm which requires a very heavy enclosure to resist the pressure difference between a 300 atmospheres external pressure and a 1 atm internal pressure. A subsea application also means that reliability of the circuit breaker is crucial and that on-site maintenance or repair brings with it unacceptably high additional cost and difficulties.